1. Field of the Invention
The present invention relates generally to rotation control circuits and, more particularly, is directed to a rotation control circuit for a Hall motor in which a rotor magnet is opposed to a plurality of stator coils and a change of magnetic field of the rotor magnet is detected by a Hall element, thereby controlling a drive signal supplied to the stator coils.
2. Description of the Prior Art
A Hall motor in which the rotational magnetic field is detected by a Hall element to control the current phase of the drive coil is generally formed as a flat brush and slotless (BSL) motor, and this type of Hall motor is useful as the power source for electronic equipment.
FIG. 1 shows an example of such a known Hall motor.
It will be seen in FIG. 1 that the Hall motor is comprised of a circular rotor magnet 1 on which north (N) poles and south (S) poles are alternately magnetized in the circumferential direction thereof, a plurality of stator coils 2a, 2b, 2c, . . . 2f opposed to the rotor magnet 1 with a predetermined clearance to generate repulsive and/or attractive magnetic fields and a plurality of Hall elements 3a, 3b and 3c located on the stator side with a predetermined spacing therebetween to detect the rotational magnetic field of the rotor magnet 1 when the rotor magnet 1 is rotated by drive magnetic fields of the stator coils 2a, 2b, . . . .
When the phase of the drive current i, supplied to each of the stator coils 2a, 2b, . . . 2f, is controlled on the basis of the phase information detected by the Hall elements 3a, 3b and 3c, the rotor magnet 1 rotates at a constant speed, rotating an optical disk through, for example, a turntable or the like which is rotatable together with the rotor magnet 1.
The optical disk, on which various kinds of signals are recorded, is rotated by the above-mentioned Hall motor or the like. In this case, if the N and S poles are irregularly magnetized on the rotor magnet 1 or if the rotor magnet is irregularly magnetized, then this leads to irregular torque of the motor so that the optical disk can not rotate regularly.
In order to solve the above-mentioned problem in the BSL motor, it is proposed to additionally provide a servo circuit which utilizes an electromotive voltage of the Hall element provided at the stator side as an FG signal, and in which the irregular rotation of the motor can be suppressed by the FG signal.
In such a servo circuit, the irregular magnetization of the rotor magnet causes a large error signal to be generated, which saturates the servo circuit (motor drive circuit). Further, when the loop gain of a servo circuit is increased to improve the response characteristic, there is then presented the problem that the servo characteristic becomes unstable.
Furthermore, in the case of the afore-noted Hall motor, the magnetic fields generated from the stator coils 2a, 2b, 2c, . . . 2f should differentially cancel each other out on the Hall elements 3a, 3b and 3c, and they should therefore not be generated as the detected signal. In practice, however, a voltage is induced by the magnetic field produced by the coils of the stator side, causing the high gain servo system to operate unstably.